This invention is directed to a haze free lubricating oil base stock.
The present invention relates to a lubricating oil base stock and to a sorption process for dehazing a base oil feed to produce the lubricating oil base stock. Lube base oils are normally prepared from crude oil distillates and residua or synthetic oils using a series of upgrading steps, which may include hydrocracking or solvent extraction to remove heteroatoms and aromatics and to increase the viscosity index of the base oil; dewaxing to remove wax; and a finishing step for stabilizing the product against oxidation and floc and color formation.
API Interchange Guidelines (API Publication 1509) defines a Group II base oil as having xe2x89xa6300 ppm sulfur, xe2x89xa790% saturates and a viscosity index of 80-120. A Group III base oil is defined as having xe2x89xa6300 ppm sulfur, xe2x89xa790% saturates and a viscosity index of xe2x89xa7120. Group II base oils are typically made using hydroprocessing (hydrocracking or severe hydrotreating) to increase the VI of the crude to the specification value. Hydroprocessing also typically increases the saturate content above 90% and reduces the sulfur below 300 ppm. Approximately 10% of the lube base oil production in the world is in the Group II category. About 30% of U.S. production is Group II. Group III base oils are typically made using wax isomerization technology to make very high VI products. Since the starting feed is waxy VGO or wax which contains mainly saturates and little sulfur, the Group III products have saturate contents above 90% and sulfur contents below 300 ppm. Fischer Tropsch wax is an ideal feed for a wax isomerization process to make Group III lubes. Only a small fraction of the world""s lube supply is in the Group III category.
Conventional methods for removing wax from a base oil feed include solvent dewaxing and catalytic dewaxing. The degree of dewaxing during one of these dewaxing processes is generally determined by the desired product pour point, where the pour point is a measurement, expressed as a temperature, at which the sample will begin to flow under carefully controlled conditions. Pour point may be determined by, for example, ASTM D5950-96. The cloud point of a lube base oil is complementary to the pour point, and is expressed as a temperature at which a lube oil sample begins to develop a haze under carefully specified conditions. Cloud point may be determined by, for example, ASTM D5773-95. Generally, both the pour point and the cloud point are decreased during dewaxing.
Typical lube base oils will have pour points below +10xc2x0 F. (xe2x88x9212xc2x0 C.) and cloud points below +14xc2x0 F. (xe2x88x9210xc2x0 C.). These specifications are satisfactory for the majority of lube base oils used in engine lubrication. For a few small volume applications intended for cold climates, lower pour and/or cloud points may be needed. The pour point, may be reduced by adding chemical pour point depressants, but these chemical additives are expensive.
Solvent dewaxing is the traditional method, but suffers from the need to use expensive solvents which also contribute to air and water emissions. Solvent dewaxing has the additional disadvantage in that it produces slack wax, which generally has low value. Catalytic dewaxing is the more recent technology and eliminates the needs for solvents. In general it is observed that oils produced by solvent dewaxing will have pour and cloud points that are not significantly different. That is, the pour-cloud spread (the absolute value of their differences) will be less than or equal to 5xc2x0 C. For example, U.S. Pat. No. 4,950,382 shows typical data from solvent dewaxing that does not exceed 5xc2x0 C. When lube base oils are produced by catalytic dewaxing, however, the pour-cloud spreads can be higher than 5xc2x0 C. As lube base oils become highly paraffinic, the pour-cloud spread tends to increase. For example U.S. Pat. No. 5,015,361 describes the preparation of a lube base oil by oligomerization of propylene in two stages. The lube base oil has a 50xc2x0 C. pour-cloud spread.
Some base oil feeds, particularly heavy streams such as bright stock, contain naturally-occurring haze precursors that are more difficult to remove by conventional dewaxing than are the paraffinic waxes which predominate in lower boiling waxy streams. If present in sufficient quantities, the haze precursors form a haze in the base oil at ambient (or lower) temperatures, particularly if the base oil is allowed to stand at the low temperature for some time, e.g. overnight. The base oil may develop a hazy appearance even after being dewaxed to a low pour point, e.g. less than xe2x88x925xc2x0 C. Conversely, the haze generally disappears when the base oil is heated slightly, e.g. to a temperature of 80xc2x0 F. or above. The haze will generally be the color of the base oil in which it forms, and is usually white when present in otherwise colorless oil. Haze precursors which give rise to the hazy appearance have significant paraffinic character, some with cyclic components having a long paraffin-like tail. As such, these haze precursors are expected to have substantially different molecular structures than do the color bodies and heteroatom molecules removed by conventional clay filtering for oil stabilization. The presence or absence of a visual haze may be determined using the clear-and-bright standard of ASTM D-4176-93 (Reapproved 1997). The haze may also be quantified by measure of clarity. Haze and turbidity are less well connected with a functional problem of the lube base oil and more associated with general customer acceptance. Customers prefer to use lube base oils that have no haze or turbidity and are xe2x80x9cBright and Clearxe2x80x9d on observation. Haze and turbidity can be present in a lube sample at temperatures in excess of the cloud point. Haze and turbidity is often associated with water, foreign solid material, and/or traces of wax-like hydrocarbons. For the subject of this invention, the cause of the haze is not associated with water or solids as these are assumed to be removed by other processing steps. The subject of this invention relates to haze and turbidity induced by traces of wax-like hydrocarbons.
While haze and turbidity are not functional problems, their importance has been recognized in the prior art. For example U.S. Pat. Nos. 4,702,817 and 4,820,400 describe the removal of haze from a lube base oil solvent mixture by electrophoresis during solvent dewaxing. In U.S. Pat. No. 4,820,400, visual inspection is used to measure the haze in the lube base oil. U.S. Pat. No. 4,627,901 describes a light-scattering turbidity measuring device for control of the electrophoresis de-hazing process.
U.S. Pat. Nos. 4,919,788, 5,110,445 and 5,302,279 mention the formation of haze when a petroleum based catalytically dewaxed oil is stored overnight. This haze is referred to as the Overnight Cloud, and is measured by D2500. These patents also describe how catalytically dewaxed lube base oils typically have higher pour-cloud spreads than solvent dewaxed lube base oils. U.S. Pat. No. 5,614,079 also confirms the higher pour-cloud spreads from catalytically dewaxed lubes and mentions the use of a relative turbidity measuring device as part of D2500.
U.S. Pat. Nos. 4,822,476 and 4,867,862 describe the use of a commercial device for quantifying the turbidity of lube base oils. In these patents, the measurement of the turbidity device is expressed as nephelometric turbidity units (NTU), and a maximum value of 24 is specified. In this test, the oil is dissolved in Methyl-Ethyl-Ketone (MEK), the wax recovered, and then redispersed in MEK where the NTU value is measured by a Hach Model 18900 ratio turbidimeter.
There are several examples in the literature that have lube base oils with satisfactory pour points (below xe2x88x9212xc2x0 C.) but with unsatisfactory cloud points (above xe2x88x9210xc2x0 C.). See for example U.S. Pat. Nos. 6,051,129, 5,413,695, 5,376,260, 5,135,638, 5,741,751, and 5,514,362. Use of catalytic dewaxing to remove the last traces of cloud, haze and turbidity is possible, but with typically a substantial loss in yield. Since cloud, haze and turbidity are predominantly non-functional specifications and associated with customer acceptance, the desired solution is one that affects the customer""s visual observation directly: haze and turbidity rather than cloud point. Thus what is desired is a lube base oil that has acceptable turbidity (below 2.0 NTU and preferably below 1.0) but can have relatively high cloud points (xe2x88x9210xc2x0 C. to +20xc2x0 C.). If a lube base oil has a cloud point above +20xc2x0 C., it will not have an acceptable turbidity. Furthermore, what is desired is an acceptable lube base oil made from a previously catalytically dewaxed lube base oil that initially had an acceptable pour point, and unacceptable cloud point, and an unacceptable turbidity. This acceptable lube base oil should be made by a process other than further catalytic dewaxing (which causes unacceptable yield losses) and not solvent dewaxing (which involves the use of expensive and environmentally undesirable solvents).
The present invention is directed to a lubricating oil base stock having a reduced tendency to form a haze after standing at ambient temperatures process, the base stock characterized by a pour point of less than xe2x88x925xc2x0 C., a cloud point greater than 0xc2x0 C., preferably greater than 5xc2x0 C. and an NTU value of less than 2.0. A preferred lubricating oil base stock has a viscosity of greater than 6.5 cSt (measured at 100xc2x0 C.), and boils in the range 650-750xc2x0 F.+, preferably in the range 750-850xc2x0 F.+. Neutral oils of the invention will generally boil at temperatures below about 1050xc2x0 F. Bright stock oils may boil at temperatures up to 1300xc2x0 F. and higher. The present invention is also directed to a lubricating oil base stock having a cloud point greater than 0xc2x0 C. and an NTU value of less than 2.0, the base stock being prepared by the method comprising contacting a 650-750xc2x0 F.+paraffinic stream derived from a Fischer-Tropsch process with a dewaxing catalyst at a temperature in the range 500-800xc2x0 F. and a pressure in the range 100-3000 psig and producing a dewaxed oil having a pour point of less than xe2x88x925xc2x0 C., a cloud point greater than 0xc2x0 C. and an NTU value of greater than 2.0 and contacting at least a portion of the dewaxed oil with a solid sorbent and producing the lubricating oil base stock.
The present invention is also directed to a process for removing a substantial portion of the haze precursors from a base oil feed, and more specifically from a dewaxed base oil feed, with little or no reduction in lube stock yield. Accordingly, the present invention provides a lubricating oil base stock and a process for producing an improved base stock with a reduced tendency to form a haze after standing at ambient temperatures, the process comprising contacting a base oil feed with a solid sorbent for a time and at conditions sufficient to produce a dehazed base oil having a reduced cloud point relative to that of the base oil feed.
Unlike conventional dewaxing processes, the present sorption process reduces the cloud point of the base oil feed with little or no effect on the yield of lube base oil. The yield of lube base oil based on the weight of base oil feed to the sorbent bed is greater than about 95%, and preferably greater than about 98%. Yields of up to 100% can be expected in some cases during steady state operation. A base oil feed which is a preferred feedstock for the present process has a low pour point, typically less than xe2x88x925xc2x0 C., though the pour point may be as low as xe2x88x9240xc2x0 C. and lower, and a pour-cloud spread of 10xc2x0 C. or more, and preferably 15xc2x0 C. or more. The cloud point of the feedstock is above xe2x88x925xc2x0 C., generally above 0xc2x0 C., and may be as high as 30xc2x0 C. or higher. In the process, the cloud point of the dehazed base oil is reduced relative to the base oil feed to the process. Preferably, the cloud point of the dehazed product from the process is less than 15xc2x0 C., and more preferably less than 10xc2x0 C.
Preferred adsorption condition for removing the haze includes a temperature in the range of 15xc2x0 C. (60xc2x0 F.) and 60xc2x0 C. (140xc2x0 F.), and a flow rate of hazy oil of between 0.01 hrxe2x88x921 and 10 hrxe2x88x921.
Further to the invention is a process for producing an improved base oil with a reduced tendency to form a haze after standing at ambient temperatures, the process comprising contacting a base oil feed, having a viscosity of greater than 6.5 cSt (measured at 100xc2x0 C.), a pour point of less than xe2x88x925xc2x0 C., and a pour-cloud spread of 15xc2x0 C. or more, with a solid sorbent at a temperature of less than 66xc2x0 C. and at a flow rate of less than 10 hrxe2x88x921 WHSV, and producing a dehazed base oil having a cloud point of at least 5xc2x0 C. lower temperature than that of the base oil feed.
Among other factors, the present invention is based at least in part on the discovery of a haze-free oil with a relatively high cloud point. Conventional dewaxed oils tend to develop a haze on standing at ambient conditions when the cloud point of the dewaxed oil is greater than 0xc2x0 C., and more readily when the cloud point is greater than 5xc2x0 C. It has been surprisingly discovered that the haze-forming tendency of a dewaxed oil may be reduced independently of the cloud point, i.e. in the process for producing the base stock of the present invention, the haze-forming tendency, as indicated by the NTU value of the base stock, is more significantly reduced than is the cloud point. Therefore, the present base oil, having a high cloud point, remains haze free, as indicated by its NTU value.